科研之友
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摘要
The 8-layered shifted hexagonal perovskite compound Ba8ZnNb6O24 was isolated via controlling the ZnO volatilization, which features long-range B-cation ordering with nanometer-scale separation by similar to 1.9 nm of octahedral d(10) cationic (Zn2+) layers within the purely corner-sharing octahedral d(0) cationic (Nb5+) host. The long-range ordering of the B-site vacancy and out-of-center distortion of the highly-charged d(0) Nb5+ that is assisted by the second-order Jahn-Teller effect contribute to this unusual B-cation ordering in Ba8ZnNb6O24. A small amount (similar to 15%) of d(10) Sb5+ substitution for Nb5+ in Ba8ZnNb6-xSbxO24 dramatically transformed the shifted structure to a twinned structure, in contrast with the Ba8ZnNb6-xTaxO24 case requiring 50% d(0) Ta5+ substitution for Nb5+ for such a shift-to-twin transformation. Multiple factors including B-cationic sizes, electrostatic repulsion forces, long-range ordering of B-site vacancies, and bonding preferences arising from a covalent contribution to the B-O bonding that includes out-ofcenter octahedral distortion and the B-O-B bonding angle could subtly contribute to the twin-shift phase competition of B-site deficient 8-layered hexagonal perovskites Ba8B7O24. The ceramics of new shifted Ba8ZnNb6O24 and twinned Ba8ZnNb5.1Sb0.9O24 compounds exhibited good microwave dielectric properties (epsilon(r) similar to 35, Qf similar to 36 200-43 400 GHz and tau(f) similar to 38-44 ppm/degrees C).
